1. Field of Invention
This invention relates to semiconductor device packages, and more particularly to semiconductor device packages employing a thermally conductive heat spreader substrate.
2. Description of Related Art
During manufacture of an integrated circuit (e.g., a microprocessor), signal lines formed upon the silicon substrate and to be connected to external devices are terminated at flat metal contact regions called input/output (I/O) pads. Following manufacture, the integrated circuit is typically secured within a protective semiconductor device package. Each I/O pad of the chip is then connected to a bonding pad of the device package, and ultimately to a terminal which typically extends from the periphery of the package.
As integrated circuit fabrication technology improves, manufacturers are able to integrate more and more functions onto single silicon substrates. As the number of functions on a single chip increases, however, the number of signal lines which need to be connected to external devices also increases. The corresponding numbers of required I/O pads and device package terminals increase as well, as do the complexities and costs of the device packages.
Grid array semiconductor device packages have terminals arranged in a two-dimensional array across an underside surface of the device package. An increasingly popular type of grid array device package is the ball grid array (BGA) device package. A BGA device includes a chip mounted upon a larger substrate made of, for example, fiberglass-epoxy printed circuit board material or ceramic material (e.g., aluminum oxide, alumina, Al.sub.2 O.sub.3, or aluminum nitride, AlN). A BGA substrate typically includes two sets of bonding pads: a first set adjacent to the chip and a second set arranged in a two-dimensional array across the underside surface of the device package. The I/O pads of the chip are typically connected to corresponding members of the first set of bonding pads by signal lines. Members of the second set of bonding pads function as device package terminals. The solder balls on the underside of the BGA device package allow the device to be surface mounted to an ordinary PCB. The substrate also includes one or more layers of signal lines (i.e., traces) which connect respective members of the first and second sets of bonding pads. During PCB assembly, the BGA device package is attached to the PCB by heating the solder balls until they reflow. When the solder cools, the substrate is physically and electrically coupled to the PCB.
Common methods used to connect the I/O pads of the integrated circuit to bond traces of the device package include wire bonding and tape automated bonding (TAB). In wire bonding, connections are made using fine metal wires (e.g., gold or aluminum wires). TAB techniques connect I/O pads of the integrated circuit to bond traces of the device package using fine-line conductors etched from one or more sheets of metal (e.g., copper). The electrically conductive sheet is bonded to a sheet of a dielectric material (e.g., polyimide film), then portions of the conductive sheet are selectively removed in order to form the TAB conductors. The sheet of dielectric material maintains proper conductor spacing during handling and installation.
Some BGA packages with enhanced thermal performance employ a thermally conductive heat spreader substrate. The heat spreader substrate may be made of, for example, a metal such as copper. The heat spreader substrate does not itself include layers of signal traces or bonding pads which function as device terminals, but provides a stiff backing for support of a flexible TAB tape which includes signal traces and bonding pads. As with standard BGA packages, an integrated circuit is mounted substantially in the center of one surface of the heat spreader. The TAB tape surrounds the integrated circuit, and a layer of an adhesive is used to bond the TAB tape to the heat spreader substrate. One end of each TAB tape conductor is connected to an I/O pad of the integrated circuit, and the other end typically terminates at a bonding pad coated with solder which functions as a device terminal.
A problem arises, however, in the bonding of the TAB tape to the heat spreader substrate. Copper readily tarnishes and corrodes, thus bare copper is not a desirable product finish. In order to provide a tarnish- and corrosion-resistant finish, copper heat spreader substrates are commonly plated first with nickel, then with gold. However, the smoothness of the outer surface of the gold plating and the chemical stability of gold cause an eventual lack of adhesion (i.e., a delamination) at the interface between the adhesive layer which bonds the TAB tape to the heat spreader substrate and the heat spreader substrate itself. Repeated heating and cooling of the device package accelerates the delamination process. The resulting physical separation between the heat spreader substrate and the TAB tape places undue stress on the delicate conductors of the TAB tape and creates a long-term device package reliability problem.
It would be advantageous to have a device package employing a heat spreader substrate and having a stable bond between the heat spreader substrate and a flexible TAB tape including signal traces and bonding pads. Such a device package would possess both improved heat transfer characteristics and long-term operational reliability.